The immune system prevents reactivity against oneself; when this fails, autoimmune disease results. In pregnancy, the immune system also protects the fetus from the mother's immune system. When a mother develops preeclampsia, her immune system is overly active. Studies have shown that her cells react to her fetus's cells more than in normal pregnancies. This may represent rejection of the fetus. Two types of immune cells are especially important in preventing reactions against oneself and the fetus. We propose to evaluate these cells; both number and function, in preeclampsia. If confirmed, problems with these cells may be targets for treatment. In addition, cells exchanged between a woman and her fetus, which is a normal part of pregnancy, contribute to the ability of the immune system to maintain tolerance during pregnancy and later in life. Additional studies aim to understand how this exchange of cells influences pregnancy outcomes.